Directional drilling systems are systems well known in the art of drilling oil and gas wellbores. Such a system generally comprises a drillstring with a bottom hole assembly (BHA) comprising a steering assembly and a drill bit attached to the bottom end of the drillstring.
In directional drilling, the bottom hole assembly generally comprises a measurement while drilling assembly (MWD) comprising sensors for measuring information about the direction (inclination and azimuth) of the wellbore and other downhole drilling parameters, and comprises telemetry transmitters for transmitting sensor data uphole to a surface control unit. Additionally, for directional control, a conventional bottom hole assembly comprises a downhole motor and bent sub coupled to a shaft for rotating the drill bit. Optionally, a rotary steerable system (RSS) may either replace or be used in combination with the downhole motor to provide steering control. The advantage of the RSS is to allow directional steering control while rotating the entire drillstring, whereas the downhole motor alone is only steerable by holding the drillstring fixed in a particular direction (or toolface) from the surface. The benefits of continuously rotating the drillstring are numerous including a large reduction in friction between the drillstring and the borehole, which permits the drilling of longer distance horizontal wells.
Rotary Steerable Systems generally comprise a tubular housing enclosing a shaft having a front end connected directly or indirectly to the drill bit. Various kinds of steering mechanisms can be included in the housing to change the orientation of the front end of the shaft to change the direction of drilling. A first category of rotary steerable systems is configured to work in a “push the bit” mode, and a second category of rotary steerable systems is configured to work in a “point the bit” mode. In push the bit mode, the bit dominant factor of steering is a side (or lateral) force imparted to the bit. In point the bit mode, the dominant factor for steering is an angular change or tilting of the bit. Each category of rotary steerable systems is comprised of further sub-categories.
For the rotary steerable systems configured to work in push the bit mode, the housing comprises pads or some other offset mechanism which can be selectively activated for applying a reactive side force on the shaft, thus changing the orientation of the drill bit.
A first sub-category of push the bit rotary steerable systems comprises a non-rotating (or slowly rotating) housing provided by a plurality of pads distributed around the circumference of the housing and directed towards the wellbore. The pads are selectively actuated to push against the wellbore formation and change the orientation of the housing which deflects the shaft and provides the required side force on the drill bit, thus deflecting the drill bit sideways in a preferred direction of drilling.
A second sub-category of push the bit rotary steerable system comprises a non-rotating (or slowly rotating) housing provided by a fixed body-mounted stabilizer and a deflection device inside the circumference of the housing and directed towards the shaft. The internal deflection device is selectively actuated to push the shaft away from the center of the stabilized housing and thus the center of the wellbore, providing a side force on the drill bit.
Another sub-category of push the bit rotary steerable system comprises a rotating housing provided by a plurality of pads distributed around the circumference of the housing and directed towards the wellbore. The pads rotate with the housing and can independently move from a retracted to an extended position, bearing against the wellbore formation and pushing the housing laterally off-center from the wellbore, thus changing its orientation. The system further comprises a control means that actuates one pad when the pad crosses a selected radial angle such that the pad pushes against the wellbore towards a selected direction to change the orientation of the housing which deflects the shaft and provides the required offset force at the drill bit. While drilling in soft formations, it may not be suitable to use a steering system which pushes pads against the wellbore, especially when rotating said pads.
For the rotary steerable systems configured to operate in point the bit mode, the primary method used to tilt the drill bit is to bend the shaft inside a centralized non-rotating (or slowly rotating) housing, thus angularly deflecting the shaft away from the centerline axis of the wellbore. In that case, the non-rotating housing includes some form of anti-rotation means and a mechanism for deflecting the shaft inside the non-rotating housing. In this case, bending while rotating the shaft can cause fatigue on the shaft, and the shaft may break or get deformed after a certain time of use. Workarounds include the use of costly materials and may require an increased shaft diameter this limiting the available cross-section for offset mechanisms, power, and instrumentation.
Beside the category of “push the bit” and “point the bit” rotary steerable systems, there also exist hybrid rotary steerable systems that are capable of steering like both a push the bit and point the bit system, depending on configuration. An example of such a hybrid rotary steerable system is disclosed in U.S. Pat. No. 7,188,685. This rotary steerable system comprises an upper section connected to a steering section and a drill bit connected to the steering section. The upper section is connected to a collar on which an upper stabilizer is provided. The steering section comprises a lower stabilizer and is connected to the upper section by a swivel which is a two degree of freedom universal joint, such that the swivel is located between the lower stabilizer and the drill bit. Pistons are located between the steering section and the upper section and are actuated to push against the steering section which pivots on the universal joint. The steering section tilts until the lower stabilizer contacts the formation at which point the pistons act to push the bit through the formation. As the formation is drilled, the constraint imposed by the formation is removed, the periphery of the steering section is allowed to tilt further and the tool then begins to steer as a point the bit system. Rotation of the steering section against the pads causes friction that can produce wear of those parts and vibration of the steering section which can influence the quality of the borehole.
It is desirable to provide a rotary steerable system that doesn't present the drawbacks of prior art devices, and which provides:                wellbore steering in either push the bit or point the bit mode;        a point the bit mode which minimizes internal cyclic bending stresses;        relatively high turn rates (or dogleg severity);        a configuration that is easily field serviceable;        the capability to vary turn rate (or dogleg severity) while providing independent directional tool face control and;        good control of the direction of drilling with less vibration.        